The present invention relates generally to low-melting point inorganic nitrate salt compositions for use as heat transfer fluids and thermal energy storage media in solar energy application, such as solar parabolic trough electrical power plant systems.
When combined with thermal energy storage (TES), a solar parabolic trough electrical power plant has the important ability to dispatch electrical output to match peak demand periods. An important component of TES system optimization is the choice of heat transfer fluids used in the TES system and/or the solar field. Very large quantities (millions of kilograms) of heat transfer fluid are required for energy storage in 100-MW to 200-MW power plants and entail high capital investment costs, so minimizing that cost while maximizing the heat transfer fluid performance is paramount.
The current generation of commercial parabolic trough plants uses a mixture of organic compounds, diphenyl oxide and biphenyl, as the heat transport fluid in the collector field. This synthetic oil currently offers the best combination of low freezing point (12° C.) and upper temperature limit (393° C.) among available heat transfer fluids. However, the characteristics of this fluid have essentially set a floor on the levelized energy costs for two reasons. First, the peak allowable fluid temperature effectively limits live and reheat steam temperatures to about 370° C., which limits the gross efficiency of the Rankine cycle. The efficiency limit sets a minimum value for the required collector area per MWe of plant rating. Second, commercial solar projects can often reduce the levelized cost of energy through the addition of a thermal storage system. The storage system, in conjunction with a larger collector field, increases the annual capacity factor and distributes the operation and maintenance costs over a larger number of megawatt-hours. However, a direct thermal storage system using organic oil as the storage medium is generally considered to be too expensive.
Current commercial projects must rely on an indirect storage system, in which thermal energy from the field is transferred to a second fluid for storage. An indirect system entails performance penalties due to the temperature drops associated with the transfer of energy between the collector loop and storage as well as the cost of collection-to-storage heat exchangers. To relax the temperature and pressure limitations of a synthetic oil, a molten salt can be adopted as the heat storage fluid. These inorganic fluids offer several favorable characteristics, including upper temperature limits in the range of 500° C., lower unit cost, vapor pressures of only a few Pascals and satisfactory physical properties. The fluid properties allow two important advancements in the technology. The Rankine cycle efficiency improves, which reduces the collector area required per MWe of plant rating and the combination of low fluid cost and low vapor pressure allows the heat transport fluid to be used directly as the thermal storage medium. The elimination of the oil-to-nitrate salt heat exchangers also reduces the unit storage system costs. Such a direct system would benefit greatly from a redesigned parabolic trough system, such as the Supertrough described by Kolb and Diver.
Sandia National Laboratories has evaluated alternative inorganic molten salts that are inexpensive relative to organic fluids, can be used at higher temperatures of 450° C. to 500° C. or more (increasing power cycle efficiency), and have virtually no vapor pressure within operating temperatures and are thus amenable to use in large TES tanks. The primary disadvantage of most molten salt formulations is relatively high freeze points that range from about 130° C. to 230° C. depending on composition as compared to about 13° C. for organic fluids. As such, considerable care must be taken to ensure salt heat transfer fluid does not freeze in the solar field or other system or TES piping. Despite these engineering issues, overall evaluations for this direct molten salt heat transfer fluid approach have been encouraging, with the main issue being selection of a molten salt formulation with an acceptable freeze point as well as high temperature durability.
Several molten salt heat transfer fluids have been used for solar thermal systems. The binary Solar Salt mixture was used at the 10 MWe Solar Two central receiver project in Barstow, Calif. It will also be used in the indirect TES system for the Andasol plant in Spain. Among the candidate mixtures, it has the highest thermal stability and the lowest cost, but also the highest melting point. Hitec HTS® has been used for decades in the heat treating industry. This salt is thermally stable at temperatures up to 454° C., and may be used up to 538° C. for short periods, but a nitrogen cover gas is required to prevent the slow conversion of the nitrite component to nitrate. The currently available molten salt formulations do not provide an optimum combination of properties, freezing point, and cost that is needed for a replacement heat transfer fluid in parabolic trough solar fields. Therefore, the work summarized in this report sought to develop an heat transfer fluid that will better meet the needs of parabolic trough plants.
Table 1 compares the liquidus temperatures (liquid-solid phase transition temperature) of a number of molten nitrate salt mixtures. Inspection of published phase diagrams revealed that ternary mixtures of NaNO3 and KNO3 with several alkali and alkaline earth nitrates have quite low melting points. The eutectic of LiNO3, NaNO3 and KNO3 melts at 120° C., while a mixture of Ca(NO3)2, NaNO3 and KNO3 melts at about 133° C. Several eutectic systems containing three constituents are liquids as low as 52° C. Unfortunately, melts containing ammonium nitrate decompose at a low temperature and those containing silver nitrate would be prohibitively expensive.
No phase diagrams appear to exist for mixtures of molten nitrate salts containing more than three constituents that would identify those having lower liquidus temperatures than the mixtures listed in Table 1. A straightforward approach to identifying an improved heat transfer fluid would be to add constituents to solar nitrate salt that depress the melting point significantly without compromising its properties. Metallic nitrates are obvious choices because of their miscibility and potential stability in contact with air. The thermodynamic criteria for melting point depression by additions of a second constituent have been summarized by Stolen and Grande. Constituents of systems whose phase diagrams that display eutectic behavior satisfy the thermodynamic criteria, while those that display peritectic behavior do not. Although the activity coefficients for all the mutual combinations of the alkali metal nitrates and alkaline earth nitrates are not available, we expect that such criteria will be satisfied by additions of calcium nitrate and lithium nitrate to sodium nitrate and potassium nitrate because all binary combinations of these nitrate salts display melting point depression, as well as eutectics, as do the ternary systems.
TABLE 1Comparison of liquidus temperatures of various molten nitrate saltmixtures. Compositions are given as mol %, cation basis. Temperaturedata were obtained from Phase Diagrams for Ceramists published byAmerican Ceramic Society/NIST.LiquidusNaKCaLiNH4OtherTemp.mol %mol %mol %mol %mol %mol %° C.Notes5050221Na—K—NO3eutectic6634238Binary Solar Salt(60-40, by wt.)74449  141Hitec ® HTSNaNO2214930133Ca—Na—K—NO3eutectic185230120Li—Na—K—NO3eutectic581131117Ca—Li—K—NO3eutectic19.329.451.392Poor thermalstability due toNH4NO320.760.219.152Lowest meltingAgNO3mixture identifiedin literatureIn this work, we investigated common-anion additions to NaNO3—KNO3 mixtures (binary Solar Salt) as a means to identify low melting (low liquidus temperature) mixtures. We evaluated the properties of multi-component molten nitrate salt mixtures as alternative heat transfer and storage fluids for a parabolic trough system. We were particularly interested in the chemical stability and viscosity of multi-component mixtures that display significantly lower melting points than Solar nitrate salt. The necessary chemical stability data regarding multi-component mixtures of alkali and/or alkaline earth nitrates were obtained experimentally.
Of course, improved molten salt compositions will be useful for a wide range of industrial process heat applications, in addition to solar thermal heat transfer and thermal energy storage systems.